Steerable trailers are well known in the art. These vehicles have wheels which turn when the tractor turns. Hydraulic sender cylinders sense the turn of the tractor and transmit this information to receiver cylinders located on one of the trailer axles causing the wheels to turn in proportion to the turn of the tractor. Steering rods connected between axles cause the wheels of other axles to turn accordingly. U.S. Pat. No. 7,686,320 describes such a steering system for a steerable trailer.
The axles of the steerable trailer are connected to the trailer frame by suspension cylinders which serve to raise and lower the frame and to cushion the trailer when it is travelling on a highway. The suspension cylinders are connected to the top of the axles.